Zener diode A zener diode can be used to maintain a fixed voltage.
Preset variable resistor A preset is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than standard variable resistors so they are sometimes used in projects to reduce the cost.
This type of variable capacitor is designed to be set when a circuit is made and then left without further adjustment.
Loudspeaker A transducer which converts electrical energy to sound.